[SciPy-user] New bmat constructor in SciPy

Nils Wagner scipy-user@scipy.net
Tue Sep 23 19:50:18 CDT 2003


------------------- 
 
One of SciPy's goals is to ease transition to Python for Matlab users. 
 
In this context it would be nice to have a tool to remove 
rows and columns of a matrix 
In Matlab: 
 
> > a=rand(4,4) 
 
a = 
 
    0.9501    0.8913    0.8214    0.9218 
    0.2311    0.7621    0.4447    0.7382 
    0.6068    0.4565    0.6154    0.1763 
    0.4860    0.0185    0.7919    0.4057 
 
use 
 
> > a(:,2)=[] 
 
a = 
 
    0.9501    0.8214    0.9218 
    0.2311    0.4447    0.7382 
    0.6068    0.6154    0.1763 
    0.4860    0.7919    0.4057 
 
> > a(2,:)=[] 
 
a = 
 
    0.9501    0.8214    0.9218 
    0.6068    0.6154    0.1763 
    0.4860    0.7919    0.4057 
 
Nils 
 
 
In that light r_[] and c_[] were added some time ago (but perhaps not  
well documented) 
 
As I mentioned before 
 
F = r_[c_[A,B],c_[C,D]]  would construct  F = [A  B] 
                                               [C  D] 
 
now, a new function has been added that does the same thing, but also  
creates a matrix object. 
 
bmat (short for build matrix) 
 
bmat([[A,B],[C,D]]) = bmat('A,B; C,D') = bmat(r_[c_[A,B],c_[C,D]]) 
 
This returns a Matrix object (* is matrix multiplication for this guy)  
constructed from the blocks A, B, C and D. 
 
Of course these blocks must have the right shapes or an error will be  
generated. 
 
Incidentally, 
 
r_['A,B;C,D']  does the same thing. 
 
 
I hope this helps. 
 
-Travis O. 
 
 
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